A bit of a disclaimer — I am a chemist, not a biologist, and my understanding of biological taxonomic classification is limited to what I learned in my high school Honors and AP Biology classes.
After puzzling for a long time over the anatomical differences between the regular kodo mounts and the epic kodo mounts, I have come to the conclusion that they represent two distinct species of kodo.
All living wild kodos and the regular kodo mount, which have the two-pronged horn on the nose, are the Lesser Kodo.
I am not sure whether the Wooly Kodos that one occasionally sees, such as the reknowned Arra’chea, represent merely a recessive phenotype or a true subspecies of the Lesser Kodo.
The epic kodo mount, which has the two rhinoceros-like horns on the nose, is the Greater Kodo.
Although the living kodos at the Kodo Graveyard in Desolace are Lesser and Wooly Kodos (examples of both types are visible in this picture), the skulls and skeletons appear to be more closely related to Greater Kodos.
Left: Greater Kodo-like skulls at the Kodo Graveyard; Right: Lesser Kodo skulls in the tent of the Thunder Bluff butcher
This suggests that the Greater Kodo may once have been the more populous species, and the reasons for its decline in the wild remain a mystery. Currently, the Greater Kodo population appears to be entirely domesticated.
The yellowed skeletons in the Kodo Graveyard all belong to Greater Kodos, but there are a few much larger, bleached white skeletons that display this unusual three-pronged nose horn and curly horns over the ears. It is not known whether these represent a now-extinct variety of kodo or if these are another type of creature entirely.
EDIT:
Rades asked about the Sunwalker Kodos. The level 20 Sunwalker Kodo is a Lesser Kodo with slightly fancier equipment than a normal riding kodo and some warpaint.
Surprisingly, the level 40 Great Sunwalker Kodo, as exemplified by Sunwalker Atohmo’s Kodo in Orgrimmar, is also a Lesser Kodo, with fancier equipment than a normal epic kodo.
The Great Brewfest Kodo is a Greater Kodo, but when I used the Brewfest Hops on the mount of a character who didn’t have Journeyman Riding yet a few years ago, her slow mount was converted to a Lesser Kodo in the Brewfest gear and colors. I didn’t test if the same result still occurs for a 60% land speed mount during this year’s Brewfest, however.
Kamalia et alia 
This is very interesting! What about the Paladin Kodos? Are they simply the Greater Kodo but with barding and accoutrements? And, I wonder if the Greater Kodos have any distant relation to the Rhinos found in Northrend. 😀
The answer to your first pair of questions demands pictures, so I have updated the post itself accordingly.
I have wondered myself if Greater Kodos are a clue that kodos and the rhinos of Northrend are related somehow. The Taunka have Lesser Kodo pack animals, but there are no wild kodos in Northrend.
I’ve gone back and forth with myself about whether kodos are reptilian or mammalian; their similarity with the clearly mammalian rhinos has previously caused me to lean toward them being mammalian, bu when I looked kodos up on WoWpedia earlier today, it said that they are reptilian.
The kodo also shares a great deal of anatomical similarity with the Thunder Lizards found in Southern Barrens, Stonetalon Mountains, and Desolace. It almost seems like the kodo is an intermediate between the clearly reptilian thunder lizard and the clearly mammalian rhino.
Wow Kamalia, this is a cool post! Need more like this 🙂
This was a great post!
Oh nice post!